U.S. patent application number 14/424491 was filed with the patent office on 2015-08-06 for carbon electrode manufacturing.
This patent application is currently assigned to RUTGERS Germany GmbH. The applicant listed for this patent is RUTGERS Germany GmbH. Invention is credited to Winfried Boenigk, Christopher Kuhnt, Henri Steinmetz.
Application Number | 20150218054 14/424491 |
Document ID | / |
Family ID | 46799102 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150218054 |
Kind Code |
A1 |
Steinmetz; Henri ; et
al. |
August 6, 2015 |
CARBON ELECTRODE MANUFACTURING
Abstract
An industrial process for the manufacture of carbon electrodes
(artefacts) comprises the steps of (a) mixing a high melting pitch
with a Softening Point Mettler (SPM) above 150.degree. C. with
carbonaceous solids at a temperature of 50.degree. C. to
120.degree. C. above the SPM of the pitch, pressing or compacting
by vibration or extrusion without intentional cooling at a
temperature close to the mixing temperature, (b) transferring the
artefacts to a carbonization furnace without intentional cooling,
(c) carbonizing the artefacts, said process does not need to cool
the pitch/coke paste after mixing and/or the green electrode after
forming, thus, the heat trapped in the green electrode can be
conserved and reduces the total energy consumption and residence
time in a subsequent carbonisation step.
Inventors: |
Steinmetz; Henri; (Aspelt,
LU) ; Boenigk; Winfried; (Ludinghausen, DE) ;
Kuhnt; Christopher; (Witten, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RUTGERS Germany GmbH |
Castrop-Rauxel |
|
DE |
|
|
Assignee: |
RUTGERS Germany GmbH
Castrop-Rauxel
DE
|
Family ID: |
46799102 |
Appl. No.: |
14/424491 |
Filed: |
August 26, 2013 |
PCT Filed: |
August 26, 2013 |
PCT NO: |
PCT/EP2013/067619 |
371 Date: |
February 27, 2015 |
Current U.S.
Class: |
264/29.1 |
Current CPC
Class: |
C04B 2235/602 20130101;
C04B 2235/6021 20130101; C04B 2235/61 20130101; C25C 3/125
20130101; C04B 2235/608 20130101; C04B 35/532 20130101; C04B
2235/422 20130101 |
International
Class: |
C04B 35/532 20060101
C04B035/532; C25C 3/12 20060101 C25C003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 31, 2012 |
EP |
12182508.7 |
Claims
1. An industrial process for the manufacture of carbon electrodes
or other carbon artefacts comprising the steps of (a) mixing a high
melting pitch with a softening point Mettler (SPM) above
150.degree. C. with carbonaceous solids at a temperature of
50.degree. C. to 120.degree. C. above the SPM of the pitch,
pressing or compacting by vibration or extrusion without
intentional cooling at a temperature close to the mixing
temperature, (b) transferring the artefacts to a carbonization
furnace without intentional cooling, (c) carbonizing the
artefacts.
2. A process according to claim 1, wherein the pitch is a coal tar
pitch or an aromatic petroleum pitch.
3. A process according to claim 2, wherein the pitch is a coal tar
pitch having a content of EPA polycyclic aromatic hydrocarbons
(PAH) of not more than 1.5%.
4. A process according to claim 2, wherein the pitch is a coal tar
pitch having a SPM of above 170.degree. C.
5. A process according to claim 3, wherein the content of EPA PAHs
is not more than 0.5%
6. A process according to claim 1, wherein the coke is a calcined
petroleum coke, a needle coke or a pitch coke.
7. A process according to claim 1, wherein the artefacts are anodes
for the alumina electrolysis.
8. A process according to claim 1, wherein said artefacts are
electrodes for steel production in electric arc furnaces.
9. A process according to claim 1, wherein the carbon body or
artefact has a weight of more than 500 kg.
10. A process according to claim 4, wherein the pitch is a coal tar
pitch having a SPM of above 180.degree. C.
11. A process according to claim 4, wherein the pitch is a coal tar
pitch having a SPM of above 200.degree. C.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The invention refers to a method for the manufacture of
carbon electrodes and other carbon artefacts.
BACKGROUND OF THE INVENTION
[0002] The predominant use of carbon anodes is to act as reducing
agent in aluminum production according to Hall-Heroult. Initially
Soederberg anodes were produced from anode pastes sintering and
carbonizing on the electrolysis cell. The capture of Polycyclic
Aromatic Hydrocarbons (PAHs) as defined by the Environmental
Protection Agency (EPA) and emitted from binder pitch when
carbonizing proved to be difficult. Today, state-of-the-art anodes
are manufactured in a carbon plant separated from the electrolysis
cells. Typically coal tar pitch with a softening point Mettler
(SPM; ISO 5940-2) of 105-120.degree. C. agglomerates petroleum
coke. Coke and pitch are mixed for that purpose at 60-100.degree.
C. above the pitch SPM. The high temperature is needed to allow the
pitch viscosity getting sufficiently low to wet the coke grains.
The resulting paste is cooled with water by about 40-60.degree. C.
and pressed or compacted by vibration to form green anodes. The
green anodes are removed from the mould, cooled with water and
stored until being packed in the baking furnace for
carbonization.
[0003] The anode manufacturing process represents a significant
part of alumina reduction costs. Even small improvements of the
overall process will have a significant effect on overall
efficiency. This explains that significant effort is invested to
increase the efficiency of anode production, as shown, for example,
in C. Lavoie, E. Bergeron, A. Proulx, "ALCAN ALMA New paste plant
start-up and early operation", Light Metals, 555-560 (2003) and M.
Kempkes, W. Meier "New concept for a green anode plant", Light
Metals, 919-922 (2008).
[0004] Still, the current process faces significant disadvantages.
The energy used to heat pitch and coke to the mixing temperature is
pulled out again at subsequent cooling before and/or after green
anode forming. Same energy has to be re-entered in the
carbonisation (baking) furnace simply blocking part of the
furnace's capacity.
[0005] It is known that in laboratory processes in the formation of
carbon electrodes no cooling steps are used before and/or after
green anode forming. This is the reason why neither in the
publication Light Metals 2011 TMS (The Minerals, Metals &
Material Society, 2011 by Winfried Boenigk, Claudia Boltersdorf,
Falk Linder, Jens Stiegert nor in U.S. Pat. No. 4,188,279 cooling
steps are described. In such laboratory processes the carbon body
has a much lower weight (lab-scale <0.5 kg; pilot-scale <6
kg) as compared to carbon bodies which are formed for industrial
scale. Carbon bodies for the manufacture of carbon electrodes for
industrial scale have a weight of more than 500 kg. This results in
a much higher force applied when transported after pressing (risk
of deformation) and a much higher internal gas pressure from pitch
volatiles (risk of expansion when leaving the press).
[0006] Water added for paste cooling prior to anode forming
evaporates. The steam is charged with pitch volatiles containing
PAHs and has to be treated for PAH removal by thermal oxidation.
Paste cooling before uniaxial pressing is necessary to avoid
sticking of the paste to the mould and to ensure mechanical
stability of the compacted paste. Hot green anodes are too weak for
handling as the binder pitch is softening.
[0007] When using vibro-compactors for anode forming the cooling by
spraying or water-bathing the green anode is needed after pressing
to ensure sufficiently high anode strength for transport and
mechanical handling. Also deformation by the load of anode layers
placed on top in the baking (carbonisation) furnace has to be
avoided. Again the heat energy trapped in the green anode cannot be
recovered.
[0008] Accordingly since carbon electrodes have been manufactured
as reducing agent in the industrial scale aluminium production
cooling steps are performed before and/or after green anode
forming. This has been an accepted process measure in the
production of carbon electrodes in industrial scale. In this regard
it is referred to Kirstine L. Hulse, Anode Manufacture (2000)
R&D Carbon Ltd. (ISBN 3-9521028-5-7). Further, it should be
mentioned that worldwide there is no industrial carbon electrode
production using a pitch with a softening point Mettler of more
than 130.degree. C.
SUMMARY OF THE INVENTION
[0009] It would be highly desirable to eliminate the cooling step
simplifying the known anode forming methods of carbon molded
articles, in particular carbon electrodes. The object of the
invention is the provision of a process which allows avoiding the
cooling steps after the mixing of the pitch/coke paste and/or after
forming of the green anode.
[0010] This object is achieved by an industrial process for the
manufacture of carbon electrodes or other carbon bodies (artefacts)
comprising the steps of
[0011] (a) mixing a high melting pitch with a softening point
Mettler (SPM) above 150.degree. C. with a carbonaceous solid at a
temperature of 50.degree. C. to 120.degree. C. above the SPM of the
pitch, pressing or compacting by vibration or extrusion without
intentional cooling at a temperature close to the mixing
temperature,
[0012] (b) transferring the artefacts to a carbonization furnace
without intentional cooling,
[0013] (c) carbonizing the artefacts.
[0014] The process of the invention does not need a cooling of the
pitch/coke paste after mixing and/or the green electrode after
forming, thus, the heat trapped in the green electrode can be
conserved and reduces the total energy consumption and residence
time in a subsequent carbonization step.
[0015] The invention is based on the finding that green anodes from
a high-melting pitch prove to be stronger than expected.
Surprisingly hot anodes made from high-melting pitches--even at a
temperature above the SPM--are extremely resistant to deformation
and can be mechanically handled without substantial deformation of
the hot green molded carbon article. For the purpose of the
invention the term artefacts comprises carbon electrodes and other
formed carbon articles.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Coal tar pitch is obtained as a product of coal tar
distillation. Pitch is a black, thermoplastic material, which
essentially includes compounds volatile above 400.degree. C. and
non-vaporizable compounds of the tar. Typical electrode binder
pitch can be obtained by vacuum distillation to SPM 105-120.degree.
C. A high-melting pitch used in the invention has a SPM of above
150.degree. C., preferable above 165.degree. C. and most preferably
above 175.degree. C. A high temperature pitch is known and
described in W. Boenigk, C. Boltersdorf, F. Lindner, J. Stiegert,
"Property profile of lab-scale anodes produced with 180.degree. C.
Mettler coal tar pitch", Light Metals, 889-893 (2011). A preferred
method to produce such high-melting pitches is described, for
example, in U.S. Pat. No. 5,262,043.
[0017] The carbonaceous solid component suitable for the process of
the invention can be calcined petroleum coke, coal tar pitch coke,
needle coke and other purified carbon carriers including recycled
carbon material (e.g. anode butts). The production of coke is known
for many years. Preferably a petroleum coke or needle coke is used
in the process of the invention. For the coke grain size the
invention has no specific requirements. Accordingly, the skilled
person will use carbonaceous solids with particle sizes as in the
known processes.
[0018] The amount of pitch in the mixture of pitch and carbonaceous
solid typically can be in the range of 10% to 20% by weight,
preferably 13% to 18% by weight, the amount of carbonaceous solids
is between 80% and 90% by weight, preferably 82% and 87% by weight,
based on the weight of the mixture.
[0019] In the process of the invention the pitch and coke
components are mixed using known processes. The mixing temperature
basically depends on the softening point Mettler (SPM) of the pitch
used in the process. Preferably the mixing temperature will be
60.degree. C. to 100.degree. C. above the SPM of the pitch.
[0020] Suitable mixing devices for the purpose of the invention are
mixers which allow to keep or to bring the pitch/carbonaceous solid
mixture to the aforementioned temperature. For the example, a
so-called EIRICH high-temperature intensive mixer, provided by
Eirich GmbH & Co. KG, Germany is particularly suited for the
process of the invention. This mixer allows mixing temperatures of
about 300.degree. C.
[0021] Alternatively the coke is preheated to a temperature above
mixing temperature allowing excess heat to be transferred to the
pitch while mixing.
[0022] The product from the mixing step is a paste which according
to the invention is not subjected to cooling as in the state of the
art processes. Thus, the hot mix (paste) can be transferred to a
mould and is compacted therein. The green body released after
compressing from the mould is placed in a baking furnace without
the cooling step of the state of the art processes at this
stage.
[0023] Also the process in the baking furnace is known. Generally
the temperature in the furnace is gradually increased to about
1100.degree. C. The hot charging of the baking furnace according to
the invention allows a faster increase of temperature in the
temperature range up to 300.degree. C.
[0024] Accordingly neither the paste obtained after mixing of the
pitch and the coke components nor the green carbon body released
from the mould is cooled with water. In the process of the
invention a water cooling step can be omitted.
[0025] The carbon bodies or artefacts obtained in the process of
the invention have a weight of more than 500 kg. In fact, they
usually have a weight of more than 1000 kg.
[0026] With reference to FIGS. 1 and 2 the advantageous properties
of the green anodes obtained according to the invention are
demonstrated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows the compression deformation [%] against delta
between testing temperature and SPM of the pitch at 1 MPa load for
several pitches.
[0028] FIG. 2 shows compression deformation [%] against delta to
SPM between 1 and 2 MPa load for several pitches.
[0029] The measured compression is depicted against the softening
point of the pitch in FIGS. 1 and 2 to allow a direct comparison.
All green anodes are rigid when cooled to minimum 30.degree. C.
below the SPM of the pitch. At the respective SPM increasing the
load from 1 MPa to 2 MPa causes a compression of nearly 1% for SPM
112.degree. C. against 0.4% for SPM 147.degree. C. whereas the SPM
182.degree. C. is still in the range of the rigid anode (<0.3%).
Further increasing of the temperature to the forming temperature
starts floating of the SPM 112.degree. C. anode, whereas an anode
from SPM 147.degree. C. reaches the 1% compression. The unexpected
resistance of the SPM 182.degree. C. anode is confirmed as it does
not exceed 0.5% compression when loading with 1 MPa and 0.55%
between 1 to 2 MPa.
[0030] Obviously the surprising effect of high rigidity of hot
green anodes starts at pitch SPM above 150.degree. C. Making use of
high-melting pitch makes it for the first time possible to produce
anodes without any cooling of the paste after mixing. The load used
in these examples exceeds the load exercised when handling anodes
between pressing and positioning in the baking furnace. Thus the
hot mix can be immediately transferred to a mould, compacted and
the green body can--again without cooling--be placed in a baking
furnace without mechanical damage. This not only provides savings
in the forming process but also helps to reduce energy consumption
in the baking furnace. When hot handling is possible, the trapped
heat can be conserved for the baking furnace significantly
improving the furnace capacity by reducing the cycle time.
[0031] Finally it can be concluded that the use of high-melting
pitch significantly improves green anode strength even at and above
the pitch softening temperature. This is the property that allows
avoiding anode cooling with all its negative implications.
[0032] The method of the invention to avoid paste cooling is
equally applicable to the production of other carbonaceous
artefacts using pitch coke, needle coke or other carbonaceous
solids.
[0033] The process of the invention is extremely advantageous
because the energy used for heating the anode paste is not lost by
cooling, no water is consumed for cooling purposes, water
contamination with PAHs is avoided, no green anode storage for
cooling is necessary, heating of the baking furnace is much quicker
as the anodes are already heated to the core, less volatiles of
high-melting pitch allow a faster carbonization cycle increasing
furnace throughput further, pitch with a low volatility improves
the occupational situation (less PAHs), high shape stability
prevents stud-hole slumping. These advantages finally provide a
better carbon footprint of the process.
[0034] The following examples serve to further illustrate the
invention. All examples are examples which are performed in
laboratory scale.
EXAMPLES
Example 1
Comparative
[0035] Green carbon anodes (50 mm o; 100-110 mm long) were produced
by mixing 84% petroleum coke (containing 20% anode butts) and 16%
electrode binder pitch SPM 112.degree. C., coking value 58.3% (ISO
6998); QI 7.7% (ISO 6791); TI 27.4% (ISO 6376) until 210.degree. C.
is reached and subsequent pressing at 600 bar. The apparent green
density is 1.695 g/cm.sup.3. The anodes are cut in pieces of 50 mm
o, 50 mm long. The cold crushing strength of the green anode is
26.4 MPa. The green anode samples were pre-heated to distinct
temperatures and subjected to controlled loading as described
below.
[0036] The load and the corresponding deformation are recorded in
Table 1. The values were obtained with a Frank Universal testing
machine (Type 81806/B, 20 kN load cell, testing speed 7.5
mm/min).
TABLE-US-00001 TABLE 1 Deformation (.epsilon.) of anodes (16%
binder pitch SPM 112.degree. C.) under loading conditions Temp.
.epsilon.(1 .DELTA..epsilon.(1-2 [.degree. C.] .DELTA. SPM MPa)[%]
.sigma. [%] MPa)[%] .sigma. [%] 20 -92 0.204 0.026 0.185 60 -52
0.220 0.015 0.227 0.014 80 -32 0.267 0.030 0.257 0.017 100 -12
0.637 0.127 0.642 0.226 110 -2 0.759 0.093 0.944 0.111 120 8 1.074
0.074 1.111 0.086 130 18 1.062 0.339 1.074 0.280
[0037] The results in table 1 demonstrate that hot green anodes are
sensitive to deformation. The compressive strain decreases as
expected when cooling whereas the anode is plastic at and above
pitch SPM. The standard deviation is significantly reduced as soon
as 80.degree. C. (.about.30.degree. C. below SPM) are reached and a
rigid anode is obtained that can be handled in the further process
steps. Just cooling the outer surface is not sufficient for
handling as the heat energy stored in the pressed green anode core
is sufficient to heat-up the shell again. These results confirm the
need for anode cooling to a minimum of 30.degree. C. below SPM. The
maximum tolerable temperature may slightly vary depending on a
plant's equipment. The tendency of a green carbon anode to bulge
not only affects the outer shape but also increases the risk of
stud-hole slumping.
Example 2
Invention
[0038] Green anodes were shaped after mixing 16% electrode binder
(SPM 182.degree. C., coking value 76.1%; QI 14.5%; TI 43.7%; no
mesophase particles detectable under polarized light), produced by
vacuum distillation at 1 mbar and 84% petroleum coke recipe
(containing 20% anode butts) to 280.degree. C. The apparent green
density is 1.694 g/cm.sup.3. The crushing strength of the green
anode at room temperature turned out to be surprisingly high at
34.9 MPa. The binding capability of this high-melting binder is
obviously considerably higher than the standard pitch tested
before. Testing of high-temperature deformation properties was
repeated with these anodes in a similar way (Table 2).
TABLE-US-00002 TABLE 2 Deformation of anodes (.epsilon.) (16%
binder pitch SPM 182.degree. C.) under loading conditions Temp.
.epsilon.(1 .DELTA..epsilon.(1-2 [.degree. C.] .DELTA. SPM MPa)[%]
.sigma. [%] MPa)[%] .sigma. [%] 30 -152 0.211 0.228 100 -82 0.209
0.028 0.219 0.023 160 -22 0.259 0.023 0.263 0.263 180 -2 0.291
0.067 0.284 0.057 190 8 0.296 0.296 200 18 0.424 0.047 0.430 0.130
230 48 0.494 0.086 0.543 0.086 250 68 0.430 0.095 0.494 0.137 270
88 0.385 0.356
[0039] Green anodes from SPM 182.degree. C. pitch prove to be
stronger than expected. Surprisingly even at a temperature above
the SPM the anodes are extremely resistant to deformation. Maximum
compression is below 0.5% (1 MPa load) even at mixing temperature.
A comparable rigidity is achieved for state-of-the-art pitches well
below their SPM thus requesting cooling. For SPM 182.degree. C.
pitch no single value exceeds the deformation level of 0.5% at 1
MPa load.
Example 3
Comparative
[0040] To find out whether pitches with SPM between 112.degree. C.
and 182.degree. C. behave in a similar way as the pitch of example
2, a pitch with SPM 146.5.degree. C. (coking value 67.6%, QI 10.5%,
TI 36.6%, no mesophase particles) was chosen. The anodes were
treated as described in examples 1 and 2. The results are presented
in Table 3. The results in Table 3 show that anodes produced using
this pitch have much less deformation resistance when heated above
the SPM compared to the pitch in Tab. 2.
TABLE-US-00003 TABLE 3 Deformation of anodes (.epsilon.) (16%
binder pitch SPM 146.5.degree. C.) under loading conditions Temp.
.epsilon.(1 .DELTA..epsilon.(1-2 [.degree. C.] .DELTA. SPM MPa)[%]
.sigma. [%] MPa)[%] .sigma. [%] 30 -117 0.278 0.241 80 -67 0.222
0.059 0.219 0.023 130 -17 0.317 0.008 0.317 0.008 150 4 0.341 0.013
0.433 0.106 170 24 0.578 0.063 0.667 0.021 200 54 0.642 0.034 0.919
0.258
[0041] The results of examples 1-3 are compiled in FIGS. 1 and
2.
* * * * *